use soundflower or conform to the visualizers protocol. if you conform to apples protocol you can play itunes with the visualizer open and get you peak and audio spectrum structures going in quartz. just remember you need the itunes visualizer open and have to be conformed to protocol in order for this to work. basically all you have to do is hook up the spectrum and peak to something in your patch from the root level in order to conform to protocol. i use soundflower until the visualizer is complete, then i go to protocol conformance and put the qtz file in the compositions repository so itunes will recognize.

I have my visualisations 'react' to audio by going through an analysis first (e.g. specific frequency energies / amplitudes) to create midi control data; which I then arrange in a sequencer, usually Ableton which feeds into Quartz Composer.

So you end up with a kind of video mixer, where the clips can be 'arranged' via the sequencer.

using SoundFlower saves a whole load of crashes that can occur if you are running iTunes to instigate what can be done standalone with audio input or better than that, SoundFlower - and it's free too :-)